1. Field of the Invention
The present invention relates to a method and an apparatus for forming a hole, in particular, a blind hole in a printed circuit board by a laser beam.
2. Description of the Prior Art
The burst processing method and the cycle processing method, which are known as a method of making a hole in a printed circuit board by a laser beam, will be described with reference to the accompanying drawings.
FIG. 7 is a view for explaining the operation based on the burst processing method, and FIG. 8 is a cross sectional view of the formed hole thereby. In the burst processing method, as shown in FIG. 7, after positioning a laser beam at a hole forming position on a printed circuit board in accordance with a processing program, N pulses (N=1, 2, 3, . . . , n) of beams, which have a definite width of pulse WC (ms), a definite peak output PC (W) and a definite period of pulse TC (ms), enough to form a hole are shot continuously. For example, in case of continuously shooting three pulses (N=3) of beams on a printed circuit board comprising a resin in 50-100 xcexcm thickness, a hole having an inlet diameter DT1 and a bottom diameter dB1 is formed by a first pulse, and thereafter the hole is made into a hole having an inlet diameter DT2 and a bottom diameter dB2 by a second pulse, and finally into a hole having an inlet diameter DT3and a bottom diameter dB3 by a third pulse, as shown in FIG. 8. That is, the printed circuit board is processed so that the hole bottom is gradually expanded. In this case, when a positioning time is set as TG (ms), the following formulas are established.
Pulse Energy: EC(mJ)=WCxc3x97PCxe2x80x83xe2x80x83(1)
Total Energy Per One Hole: EN(mJ)=ECxc3x97Nxe2x80x83xe2x80x83(2)
Processing Period Per One Hole:
TD(ms)=TG+TCxc3x97(Nxe2x88x921)+WCxe2x80x83xe2x80x83(3)
FIG. 9 is a view for explaining the operation based on the cycle processing method, and FIG. 10 is a cross sectional view of the formed hole thereby. In the cycle processing method, as shown in FIG. 9, a single beam which. has a definite width of pulse WC (ms) and a definite peak output PC (W) is shot at each of a plurality of hole forming positions in accordance with a processing program for forming a hole in progress. Subsequently, the program is repeated for a plurality of cycles (N cycles) until a required hole shape is formed, so that the required holes are formed. For example, in case of forming a hole so as to gradually expand the hole bottom by repeatedly applying the process for three cycles to a printed circuit board comprising a resin in 50-100 xcexcm thickness, a hole having an inlet diameter Dt1 and a bottom diameter db1 is formed by the pulse in a first cycle, and thereafter the hole is made into a hole having an inlet diameter Dt2 and a bottom diameter db2 by the pulse in a second cycle, and finally into a hole having an inlet diameter Dt3 and a bottom diameter db3 by the pulse in a third cycle, as shown in FIG. 10. As is apparent from the description mentioned above, pulse energy EC and total energy EN per one hole in the cycle processing method are the same as those of the burst processing method, however, a processing period TD per one hole is expressed by the following formula (4).
TD(ms)=(TG+WC)xc3x97Nxe2x80x83xe2x80x83(4)
As mentioned above, in both of the burst processing method: and the cycle processing method, the shorter the beam positioning time TG and the pulse period TC is, or the smaller the number N of pulse shot is, the faster the hole forming speed becomes. However, in the cycle processing method, since the hole to be shot is different at every one pulse, there is a disadvantage that the hole forming speed is reduced to almost 1/N in the case that the number of cycle becomes large. For example, in case of using a galvano mirror for positioning the beam, the practical positioning period of the beam is about 2 ms (frequency 500 Hz). Moreover, the pulse period capacity of an oscillator is about 0.5 ms (the frequency of 2000 Hz). Furthermore, taking the hole quality into consideration, three pulses per one hole are required in the case that the pulse width is 15 xcexcs and the pulse period is 2 to 1 ms (the frequency of 500 to 1000 Hz). Accordingly, the practical processing period per one hole in the burst processing method is 6.02 to 4.02 ms (the hole forming speed is 166 to 248 holes per second). On the contrary, the hole forming period in the cycle processing method is 6.05 ms (the hole forming speed is 165 holes per second).
In case of the burst processing method, if the pulse period TC is reduced in order to increase the processing speed, plasma will be easily generated in a hole by the second pulse or the pulse thereafter, since decomposed materials are exposed by the following beam without being sufficiently scattered. If the plasma is generated, a wall surface of the hole is formed in a barrel shape, that is, a center portion of the hole in the depth direction is expanded in the diametrical direction, so that the hole can not be formed in a tapered shape, or the energy becomes in short for a decomposition threshold because of the energy loss due to the generation of the plasma, so that the hole is only carbonized and can not be expand. Accordingly, depending on materials, it may be necessary to increase the pulse period to 5 ms, and reduce the pulse width to 15 xcexcs for increasing the number of shot by one or two shot. In this case, the processing speed is widely reduced. Further, if increasing the pulse width WC for improving the processing speed, although the residual resin debris in the hole bottom is reduced, the energy is consumed for increasing the temperature of the inner layer and the periphery of the hole, so that the quality of the hole is bastardized. Further, since the energy reaching a copper foil surface of the hole bottom increases rapidly, the temperature increase of the hole bottom becomes high, so that there is a case of melting the inner layer copper foil or melting out to pass through the foil.
On the contrary, in case of the cycle processing method, since the pulse period with respect to one hole is sufficiently long, and the hole forming by the following pulse is performed after the decomposed materials generated by the first pulse are sufficiently scattered, the plasma is less generated in the hole in comparison with the burst processing, the hole shape is better than that of the burst processing method, and a barrel-shaped inner wall of the hole and an inferiority of the hole quality due to the insufficiency of energy are less generated. Further, since the diffusion of heat is improved and the temperature increase is reduced, the melt and the passing-through of the inner layer copper foil are reduced. However, the processing speed is low. Then, even in the cycle processing method, as shown in FIG. 11, there is a case that the curved resin is left in the side close to the hole bottom although it is intended to form the diameter of the bottom portion into dB4 (or db4), so that the inferior plating is generated in the later step or it is impossible to secure an area for turning on electricity.
An object of the present invention is to provide a method and an apparatus of making a hole in a printed circuit board, which can solve the problems mentioned above, and improve the quality of the hole shape and the processing speed.
In order to achieve the object, in accordance with the present invention, there is provided a method of making a hole in a printed circuit board, comprising a first step of forming a hole reaching a bottom, a second step of expanding the diameter of the hole bottom, and a third step of finishing an end portion of the hole bottom. Moreover, in accordance with the present invention, the laser beam energy in the second step may be smaller than the laser beam energy in the first step and the laser beam energy in the third step is smaller than the laser beam energy in the second step. Furthermore, the laser beam energy may be changed in each of said steps by changing the pulse width while fixing the peak output value of the laser beam. It is not necessary to employ the same processing method in each of the steps, for example, the cycle processing may be performed in the first and second steps and the burst processing may be performed in the third step.
In accordance with another aspect of the present invention, there is provided an apparatus of making a hole in a printed circuit board, which has a control apparatus for performing the control mentioned above. The control may be achieved by controlling a laser beam oscillator and a galvano mirror.